An aircraft can be propelled by a number of turbojets each housed in a nacelle also accommodating a collection of auxiliary equipment associated with its operation and performing various functions when the turbojet is operating or stationary.
A nacelle such as that shown by way of example in FIG. 2 generally has a tubular structure comprising
(i) an air inlet 20 in front of a turbojet 30,
(ii) a mid-section 40 intended to surround a fan of the turbojet,
(iii) a rear section 50 which may optionally incorporate thrust reversal means and which is intended to surround the combustion chamber of the turbojet 30, and
(iv) an exhaust nozzle 60 whose outlet is situated downstream of the turbojet.
Modern nacelles are often intended to accommodate a bypass turbojet which is designed, via the rotating fan blades, to generate a hot air flow (also known as primary flow) from the combustion chamber of the turbojet.
Each propulsion unit of the aircraft is thus formed by a nacelle and a turbojet, and is suspended from a fixed structure of the aircraft, for example below a wing or on the fuselage, by means of a pylon or strut attached to the turbojet or to the nacelle.
The rear section of the outer structure of the nacelle is usually formed by two cowls 51 of substantially hemicylindrical shape, on either side of a longitudinal vertical plane of symmetry of the nacelle, said cowls being movably mounted so as to be able to deploy between a working position and a maintenance position in order to give access to the turbojet.
The two cowls 51 are generally mounted pivotably about a longitudinal axis forming a hinge in the upper part (at 12 o'clock) of the reverser. The cowls 51 are maintained in a closed position by means of locks arranged along a junction line situated in the lower part (at 6 o'clock).
A 6 o'clock, the turbojet has an offset piece of equipment; this is an accessory gearbox which particularly comprises a starter, a fuel pump and a hydraulic pump. A power shaft connects the starter to the turbojet, and various lines, particularly fuel and pressurized fluid lines, are arranged radially in order to supply the turbojet from the accessory gearbox.
On account of the presence of fuel, the region of the nacelle that provides the connection between the accessory gearbox and the turbojet must be confined so that any fire possibly arising through a leak of fuel does not propagate to the rest of the propulsion unit.
Fire-resistant seals are usually composed of a silicone matrix, which imparts elasticity to the seal, reinforced with glass or carbon fabric which provides fire resistance. The fire resistance is mainly obtained by the glass or carbon fabric, the weave of which is dense and tight. On the other hand, this type of seal is extremely stiff and is incapable of hugging an irregular bearing surface. In particular, this type of seal is inappropriate for curvilinear regions having a small radius. Now, a nacelle is a complex component which has particularly irregular internal contours.
In addition to constituting a fire barrier, a seal must produce a sealing barrier between the nacelle and the turbojet, that is to say that it must permanently maintain contact between these two elements. Now, in an aircraft propulsion unit, the nacelle may have large dimensions and, therefore, the relative movements which are produced between the nacelle and the turbojet during its operation may be of large amplitude.